There have been various photoelastic strain gauges which have been commercialized and/or the subject of patents. Such gauges are characterized by having a body of photoelastic material which may be in the form of a plate or a slab, for example, and have one of a variety of shapes, such as, rectangular or cylindrical. Photoelastic material has the optical properties of polarizing light when under stress and then transmitting such light on the principal stress planes with velocities depending on the stresses, the latter being known as birefringence. The photoelastic material is clear, elastic and should be homogeneous, optically isotopic when under no stress or strain, and reasonably free from creep, aging and edge disturbance. Examples of photoeleastic material are Bakelite, celluloid, gelatin, synthetic resins, glass, and other such commercial products that are optically sensitive to stress and strain.
When the gauge is subject to monochromatic polarized light, the birefringence of the photoelastic material causes the light to emerge refracted in two orthogonal planes. Because the velocities of light propagation are different in each direction, there occurs a phase shifting of the light waves. When the waves are recombined with polarizing film, regions of stress where the wave phase is canceled appear black, and regions of stress where the wave phase is combined appear light. When white light is used in place of monochromatic light, the relative retardation of the photoelastic material causes the fringes to appear in colors of the spectrum.
There are a number of extant patents found in this broad category of photoelastic stain gauges, such as: U.S. Pat. Nos. 2,985,748 issued to Mendelsohn; 3,034,395 issued to Zandman; 3,067,606 issued to Oppel; 3,074,271 issued to Redner; 3,077,813 issued to Zandman; 3,198,063 issued to Redner; and 3,216,312 issued to Oppel. Each of these patents discloses a photoelastic strain gauge which displays visual patterns representative of the strain in a workpiece to which the gauge is bonded. One skilled in the "reading" of such gauges can evaluate quantitatively and/or qualitatively, the stress in the workpiece. Such prior art devices are quite adequate for evaluating the relatively large value stresses that are usually encountered. However, in certain instances, when the stress is minute, for example, on the order of 10.sup.-5 these prior art gauges are deficient. Specifically, the visual patterns produced as the result of such small stresses are virtually indetectable.